1. Field of the Invention
This invention relates to an electrophotographic photosensitive member having a specific surface layer thereupon, a process cartridge containing the electrophotographic photosensitive member, and an image forming apparatus.
2. Related Background Art
The surface of an electrophotographic photosensitive member must be sufficiently durable, because mechanical and electric forces involved in the operation of a charging means, developing means, transferring means and cleaning means are often imposed upon it from outside.
To be more explicit, the surface must be sufficiently durable to withstand wear and damages due to friction, and deteriorating effects by ozone often generating in association with corona charging at high humidity. Further, it is a problem that toner adheres to the surface of the photosensitive member due to repeated developing and cleaning. Therefore, the surface of the photosensitive member must be provided with improved cleaning property.
To provide the surface of the photosensitive member with properties to cope with above problems, surface protective layers containing various resins as their main ingredients on the photosensitive member have been tried. For example, Japanese Patent Application Laid-Open No. 57-30843 proposes a protective layer whose resistance to wear and electric resistance are controlled by the addition of metal oxide particles to act as electro-conductive particles.
Besides, studies have been made to improve the physical properties of the surface of the photosensitive member by adding various materials thereto. Such materials include, to take silicone compounds as an example which have been known to have a low surface energy, silicone oil (Japanese Patent Application Laid-Open No. 61-132954), polydimethylsiloxane, silicone resin powders (Japanese Patent Application Laid-Open No. 4-324454), cross-linked silicone resins, poly(carbonate-silicon) block copolymers, silicon-modified polyurethanes, and silicon-modified polyesters.
The representative polymers which have a low surface energy include fluorine polymers which are represented further by polytetrafluoroethylene powders and carbon fluoride powders.
A surface protective layer comprising a metal oxide or the like tends to have a big surface energy while having a sufficient hardness, and thus it may cause problems of the cleaning property. The silicone resin, though being excellent in having a small surface energy, is not readily compatible to other resins. Therefore, when such a resin is used in an addition system, it tends to agglutinate to cause light scattering, or to bleed upon the surface to locally crystallize there, thereby impairing the stability of the product. The fluorine polymer which is known to have a low surface energy is usually insoluble to solvents and has a poor dispersability. Therefore, the surface of a photosensitive member made from the fluorine polymer may be short in lubricity or smoothness, and, having so small a refraction index as to cause light scattering, in transparency. Further, as the fluorine polymer is usually soft, it is susceptible to mechanical damages.